This invention relates to capacitors and in particular concerns surface mount capacitors that can be connected directly to printed metallic traces.
Small dielectric chip capacitors are frequently used in microwave and millimeter wave circuits and are also employed as a DC block for passing a signal. There are several simple designs for passing signal from one printed circuit trace to another, but to date, these have been characterized by significant insertion losses at certain frequencies, and have been difficult to attach automatically to a printed circuit board. One approach has been to use a standing dielectric chip with metallized surfaces, with metal leads attached to the respective surfaces. The capacitor has to be held in place while the leads are soldered to the respective traces. Because the leads have a finite inductance, this approach typically has significant signal resonances, especially for frequencies above a few gigahertz. Also, the leads can break or fail. Another approach is a flat horizontal capacitor with the metallization on its lower side having a gap. A device of this type is the Gap-Cap, manufactured by Dielectric Laboratories, Inc. These devices can also create unwanted resonances at frequencies above a few gigahertz. In addition, these capacitors are all quite small, typically about 20 to 25 mils (0.5 to 0.6 millimeters). At these sizes, the capacitors are difficult to handle, and must be installed using a microscope. There is a definite top and bottom, and it is crucial to install them in the proper orientation, to avoid shorting the circuit. Also, in the case of the capacitor, the requirement to attach metal leads can make their manufacturing costs prohibitive.
The small size of the capacitor is necessary to match the metal film traces as nearly as possible. While it is possible to employ wider bonding pads to accommodate capacitors of a larger physical size, the bonding pads also have to be matched to a transmission line, and any change in width from the bond pad to the transmission line can also result in undesirable insertion loss.